Digital switching of communication connections through an exchange, such as a telephone exchange, has in principle been possible for a long time, but has not been useful in practise. The reason is that digital switching networks should be controlled by data processing units in order for the networks to be fully utilized. Recent developments have made the data processing units so cheap that the digital switching technique is now expected to become extensively used in the years ahead.
As telephone exchanges become very complex the error finding and correcting means gain in importance, and the present invention therefore relates to such means.
It is known to use two redundant switching networks which are controlled by two redundant data processing units. One of the processing units is active and controls the two switching networks, while the other is idle and ready to take over the control in case of errors in the active units. The units work in parallel which permits comparisons to be made between the output signals on the main busses of the units for purposes of error finding. This error finding technique requires, however, so-called microsynchronization between the processing units, which is partly a cumbersome technique and prevents the use of more than two units for controlling the switching networks with a view to increasing the capacity of the telephone exchange.
Thus the invention particularly concerns digital telephone exchanges, which means that the switching network of the exchange is a digital network that is controlled by data processing units, the subscriber circuits being based on either digital or analog techniques. In the latter case analog/digital conversion is used between the subscriber lines and the exchange. The description of the invention may therefore be restricted to data processing unit controlled switching circuits and associated PIMs (Pulse Code Modulated Interface Modules).